The present invention relates to the field of preparing polychromatic printing plates, and particularly to a method for reducing or eliminating defects caused by misregistration of the polychromatic printing plates.
In polychromatic printing using subtractive color mixing, an image is reproduced by three or four printing inks which are printed on top of each other. Thus, in order to produce the spectrum of colors, the appropriate amounts of the three or four printing inks for printing cyan (C), magenta (M), and yellow (Y), and frequency black (K), are placed upon each image point, preferably in that order. The printing ink may include other colors, such as gold, silver, green, etc., called "special" colors. The amounts of the printing inks which are determined by printing ink signals, are defined as color separation signals.
Misregistration of polychromatic printing plates does not occur when printing a regular scanned picture area, because the pictures are scanned through an unsharp mask and therefore the scanner normally blurs the edges between different colors. However, misregistration problems can occur in the press process and is particularly noticeable between flat areas, or combinations of flat areas and picture areas, in some of the separations. Flat areas in Color Electronic Prepress Systems (CEPS) contain the line work (i.e., work in which there are running lengths of color elements of the same value), as distinguished from continuous tone areas (areas in which the color elements change in value from one element to the next).
For this reason, most of the EPC systems today have a "frames" process, in which the system generates a frame between bordering colors that were predefined by the user. This requires the user to define a plurality of parameters, such as: the bordering colors to be framed, the direction of framing for each pair, and the percentages in each color separation that will create the new color. Such a procedure requires not only considerable time to execute, but also considerable expertise and experience by the operator and is open to human errors and misjudgements.
One method in use for generating the frames automatically frames each color area according to the color separations that it contains. Thus, the luminance (i.e., darkness) of each color separation is determined, and the framing is effected, according to the color separation having the highest luminance, the other separations being shrunk with respect to the corresponding separations of the other color. Such a method, however, is not entirely satisfactory since it does not take into consideration the bordering colors of each color pair, or the percentages in each separation For example, if one color consists of the separations cyan and magenta, and the other color consists of cyan and black, when the printing is effected according to the sequence C,M,Y,K, a white line will occur in case of misregistration between the magenta and the black.
Another method is described in Hennig et al. U.S. Pat. No. 4,583,116. In this system, the four process color separations (C,M,Y,K) are weighted by multiplying the color area of each separation by its respective darkness coefficient, and the darker separation in each color area defines the darkness of the color area. The framing is then effected with respect to all the color pairs by spreading the lighter (lower luminance) separations from the lighter area (other than the darkest determining the darker areas) towards the corresponding separations of the darker (higher luminance) area. However, this method also does not always produce satisfactory results because in many case the overall darkness of a color is significantly influenced not only by the darkest separation but also by the other separations of the respective color.